Hello again, content writer! Meet the first principle of WCAG 2.0. (Remember, that stands for Web Consortium Accessibility Guidelines, and we're only going to look at the guidelines that apply to you.)

Principle 1: Perceivable - Information and user interface components must be presentable to users in ways they can perceive.

Right, do you get it? Anything you put on a web page must be "perceivable": so if people can't see it, they must be able to get the information some other way. Here comes the very first guideline for you as a writer:

Guideline 1.1 Text Alternatives: Provide text alternatives for any non-text content so that it can be changed into other forms people need, such as large print, braille, speech, symbols or simpler language.

Plain text is no problem. It's the ultimate in accessible content. The words you write are usually published in some variety of good old HTML, and truly HTML is both good and old. It's so basic, it can be changed many ways without much trouble—for instance, into an audio version that people can listen to on their iPod. Consequently most people can get text information even if they:

are deaf or blind, or

use an unusual old computer or browser, or

want to listen instead of read.

So, that's OK then. Except when you include something other than words in your content—for example, a photograph or drawing or other image.

Most people can "read" the visual signal of a green light. But if you're colour blind, your passenger might give you a verbal alternative to the green light, which you don't recognise. That's what you, the content writer, must do with every image you add to the content.
Think what information it provides. Then provide the same information (briefly) in words.

If you're using a content authoring system, it's sure to have a field labeled "ALT-TEXT". That's where you write your little description of the image, obviously.

Nobody else can write your ALT-text. You're the only one who knows why you've added that image, and what information you want it to convey. Take this picture of a house and tree:
You may be writing about tree surgery, and intend the image to convey: "House with overhanging tree".

But the web team doesn't have time to read every bit of content they upload. So they may write instead: "Family home with established garden" or "4-bedroom colonial cottage".

Some images, like graphs and diagrams, are far too complicated to explain in a few words of ALT-text. Then you need to write a text equivalent, so that people "perceive" certain information. Not all the information, but the essential information.

Some other non-text information is too complicated even for a text equivalent! No words can adequately replace them.

In those cases, you still need to provide ALT-text, but just enough description so that people can tell what they are missing.

Examples:

A test that would be invalid if presented in text alone.

An image intended to create a specific sensory experience.

Media that takes time to play out. (More about media later.)

And some images don't have any meaning at all! They're just there as decoration. In that cases, don't waste readers' time by explaining: they're not missing much. Anyway, those images are generally not your problem. They still need empty ALT-text code, but it's unlikely that you, the writer, would want to add these fancy touches to your content (or be allowed to).